Irrigation requirement of onion depends upon the season, soil type, method of irrigation and age of the crop. In general, onion needs irrigation at the time of transplanting, three days after transplanting and subsequently at 7-10 days interval depending upon the soil moisture. In general, Kharif crop needs 5-8 irrigations, the late Kharif crop requires 10-12 and Rabi crop needs 12-15 irrigations. Onion being a shallow rooted crop, needs frequent light irrigation to maintain optimum soil moisture for proper growth and bulb development. Irrigation needs to be stopped when the crop attains maturity (10-15 days before harvest) and the top starts falling which helps in reducing the rotting during storage. Excess irrigation is always harmful and dry spell followed by irrigation will result in the splitting of the outer scales and also formation of bolters. Water loss with flood irrigation is too high due to conveyance, seepage and percolation losses.
Modern irrigation techniques such as drip and micro sprinkler irrigation help in saving irrigation water and improve the marketable bulb yield significantly. In case of drip irrigation, seedlings need to be planted at a spacing of 10 x 15 cm in a broad bed furrow (BBF) of 15 cm height and 120 cm top width with 45 cm furrow. Each BBF should have two drip laterals at (16 mm size) 60 cm distance with inbuilt emitters. The distance between two inbuilt emitters should be around 30-50 cm and the discharge flow rate is 4 l/hr. In case of micro sprinkler, the distance between two laterals (20 mm) of micro sprinkler should be 6m with a discharge rate of 135 l/hr. The research outcome indicated that the drip irrigation at 100 % Pan Evaporation (PE) significantly improved the marketable bulb yield (15-25%) with higher per cent A grade bulbs, water saving of about 35-40% and labour saving of 25-30% as compared to flood irrigation.
Fetigation is an effective and efficient method of applying fertilizers through drip irrigation which is used as the carrier and distributor of irrigation water and crop nutrients. Application of fertilizers @ NPK 40:40:60 kg /ha as basal and the remaining 70 kg N in seven splits through drip irrigation is recommended for achieving higher marketable bulb yield and cost benefit ratio. The drip irrigation system not only helps in water saving but also reduces nitrogen losses by leaching into ground water, as in fertigation, fertilizer nutrients are applied in root zone only.